A tornado watch has been issued for an area just west of the city of Chicago—including Lee, DeKalb, Ogle, Kane and McHenry Counties----until 12 midnight. The weather situation unfolding Thursday in the Chicago area and across a wide swath of the Midwest is a potentially dangerous one and threatens a powerful one-two meteorological punch---powerhouse 50+ mph non-thunderstorm wind gusts this afternoon and evening followed by strong, possibly severe thunderstorms, which could include scattered tornadoes, over at least sections of the metro area Thursday night. Non-thunderstorm straightline winds have been raking the area for hours and have already downed tree limbs and created damage. Peak velocities as recorded at 47 of the 153 Chicago metro area WeatherBug wind sensors have already exceeded 45 mph—including a gust of 51 mph at the LaSalle Bank building in downtown Chicago, 48 mph East Chicago and a number of sites within the Chicago city limits proper, and 45 mph Buffalo Grove, 44 mph at Gary and Hammond, IN, Kankkee and Oak Lawn, Il. The Harrison-Dever Crib three miles off Chicago Lake Michigan shoreline has been hit with 60 mph gusts. The winds sweeping the Chicago are projected to grow stronger this afternoon by a series of computer models, gusting to 50+ mph in additional areas before the afternoon and evening conclude.
Of equal and perhaps greater concern is the potential for damaging—in some cases possibly tornadic----thunderstorms Thursday night. As always, these storms may hit one area harder than another. But powerful thunderstorms may sweep at least sections of the greater Chicago metro area---which we are defining quite broadly as the the area extending from DeKalb and Rockford on the west to LaPorte and Rensselaer, IN to the east and south between 7 p.m. and 3 a.m.---arriving earliest at the western end of the metro area first then reaching eastern sections of that area late in the 7p.m. to 3 a.m. time frame. It’s possible few blustery t-storms may pop up in later this afternoon or early evening in advance of the primary severe weather period. But, a “cap’---a warm air layer aloft which interferes with updrafts of air which would otherwise produce t-storms in the hot 90-degree ground-level air mass currently in place---is likely to initially quell a large number of storms from forming in the afternoon and early evening period.
Wind advisories cover the entire Chicago metro area through midnight and the Storm Prediction Center has issued a rare “moderate” to “high” risk assessment as wekll. Sections of 25 states are under one form of weather advisory or another related to a mammoth storm draped across the Nation’s Heartland and centered in North Dakota.
Driving all features of Thursday’s wild weather is what amounts to a winter-intensity storm with a 978 mb (28.88”) central pressure superimposed of a hot, humid and therefore energy-rich summerlike air mass. The two don’t mix well and can encourage explosive severe weather development. The heat and humidity provide the fuel for thunderstorms at the same time increasing the rate at which temperatures decline with height. The heated air grows buoyant in this environment and rises forming towering cumulonimbus clouds (thunderheads) capable of gushing powerful straightline winds down to the surface and spinning up potent twisters. Meantime, the shear (wind speed and directional changes) created by powerful jet stream winds end up rotating the storms, creating supercells (rotating thunderstorms) in the process.
By definition, a “moderate” risk assessment from the Storm Prediction Center, in place across an area from eastern Oklahoma north across all of Wisconsin and the U.P. and extending into western sections of Lower Michigan—the entire Chicago area is included----Thursday night, suggests the threat of at least 30 reports of 1+” diameter hail and the potential for 6 to 19 reports of tornadoes somewhere in the outlooked area. The high risk area from north-central and northwest Illinois (including an area from Rockford west to Galena and Dubuque) and much of the eastern three quarters of Wisconsin is threatened with at least 20 tornadoes—including two of potential Enhanced Fujita Scale ratings of 3 or greater (indicating vortex winds of 136-165 mph).
The one wildcard in this severe weather situation—and it’s not insignificant—is the warm air aloft—the so-called “cap”. Computer models break this down, allowing severe storms to develop. Should the cap prove stronger than currently predicted, storms wouldn’t occur in the numbers of intensities feared in a worst-case scenario. Future weather statements should be monitored carefully. As of the 3p.m. posting of this update, intense t-storms are developing in a series of lines across Iowa, Missouri, northeast Iowa and western Wisconsin Doppler Scanned to heights of 60,000 ft.
Issued 3p.m. Thursday afternoon
Tom Skilling
WGN-TV Chief Meteorologist
